Audio compressors are well known devices which are used to modify the dynamic range of an audio signal. An audio compressor may be considered as having two parts: an electronically controlled variolosser or gain adjusting device and a control system with associated circuits to generate control signals which control the gain of the device as a prescribed function of the input (or output) signal. Variolossers are also useful in other applications. For example, a variolosser may be used as a fixed gain device such as a preamplifier or filter with slight modifications.
Audio compressors are characterized in two categories. If the control signals are derived from the input signal of the compressor, the compressor is sad to be of the feedforward type; if the control signals are derived from the output of the compressor, the compressor is said to be of the feedback type. The feedforward configuration requires that the dynamic range of the gain control circuitry be equal to that of the input signal. The advantage of this configuration is that the circuit is inherently stable. In a feedback configuration, a larger range of signals can be accurately processed since the gain control signal samples the output signal which has already been compressed. However, instability is often a problem with feedback compressors. In either of the above cases, a derived control voltage is used to control the loss or gain of the variable loss circuit.
Because of the trend toward miniaturization in electronics today, many audio devices are being designed to operate with single cell batteries. In the field of audio signal processing, a need exists to manufacture compressors, expanders, amplifiers, and filters which may be operated at very low voltages. Low voltage applications for these devices are numerous. However, because of the complexity of the functions provided by compressors and expanders it has previously been impossible to manufacture these circuits in a topology which functions reliably at power supply voltages as low as 1 volt and provide a continuously variable compression ratio. For an example of a low voltage compressor circuit employing the variolosser of the present invention, refer to U.S. patent application No. 159,949 invented by the present inventors and filed of even date herewith.
Operation at low voltage allows a compressor or expander to be powered by a single cell battery. However, operation from a single cell battery severely limits the amount of total power available to the circuitry powered therefrom. Therefore, it is also desirable to design low voltage circuitry wherein the current consumption of the circuitry is minimized.